Not Applicable
The present invention relates to carbon foam, and more particularly to a process and apparatus for extruding a thermally conductive carbon foam.
The extraordinary mechanical properties of commercial carbon fibers are due to the unique graphitic morphology of the extruded filaments. See Edie, D. D., xe2x80x9cPitch and Mesophase Fibers,xe2x80x9d in Carbon Fibers, Filaments and Composites, Figueiredo (editor, Kluwer Academic Publishers, Boston, pp. 43-72 (1990). Contemporary advanced structural composites exploit these properties by creating a disconnected network of graphitic filaments held together by an appropriate matrix. Carbon foam derived from a pitch precursor can be considered to be an interconnected network of ligaments or struts. As such interconnected networks, they would represent a potential alternative as a reinforcement in structural composite materials.
Recent developments of fiber-reinforced composites has been driven by requirements for improved strength, stiffness, creep resistance, and toughness in structural engineering materials. Carbon fibers have led to significant advancements in these properties in composites of various polymeric, metal, and ceramic matrices.
However, current applications of carbon fibers has evolved from structural reinforcement to thermal management in application ranging from high density electronic modules to communication satellites. This has stimulated research into novel reinforcements and composite processing methods. High thermal conductivity, low weight, and low coefficient of thermal expansion are the primary concerns in thermal management applications. See Shih, Wei, xe2x80x9cDevelopment of Carbon-Carbon Composites for Electronic Thermal Management Applications,xe2x80x9d IDA Workshop, May 3-5, 1994, supported by AF Wright Laboratory under Contract Number F33615-93-C-2363 and AR Phillips Laboratory Contract Number F29601-93-C-0165 and Engle, G. B., xe2x80x9cHigh Thermal Conductivity C/C Composites for Thermal Management,xe2x80x9d IDA Workshop, May 3-5, 1994, supported by AF Wright Laboratory under Contract F33615-93-C-2363 and AR Phillips Laboratory Contract Number F29601-93-C-0165. Such applications are striving towards a sandwich type approach in which a low density structural core material (i.e. honeycomb or foam) is sandwiched between a high thermal conductivity facesheet.
Structural cores are limited to low density materials to ensure that the weight limits are not exceeded. Unfortunately, carbon foams and carbon honeycomb materials are the only available materials for use in high temperature applications ( greater than 160xc2x0 C.). High thermal conductivity carbon honeycomb materials are extremely expensive to manufacture compared to low conductivity honeycombs, therefore, a performance penalty is paid for low cost materials.
Typical foaming processes utilize a xe2x80x9cblowingxe2x80x9d technique to produce a foam of the pitch precursor. The pitch is melted and pressurized, and then the pressure is reduced. Thermodynamically, this produces a xe2x80x9cFlash,xe2x80x9d thereby causing the low molecular weight compounds in the pitch to vaporize (the pitch boils), resulting in a pitch foam. See Hagar, Joseph W. and Max L. Lake, xe2x80x9cNovel Hybrid Composites Based on Carbon Foams,xe2x80x9d Mat. Res. Soc. Symp., Materials Research Society, 270:29-34 (1992), Hagar, Joseph W. and Max L. Lake, xe2x80x9cFormulation of a Mathematical Process Model Process Model for the Foaming of a Mesophase Carbon Precursor,xe2x80x9d Mat. Res. Soc. Symp., Materials Research Society, 270:35-40 (1992), Gibson, L. J. and M. F. Ashby, Cellular Solids: Structures and Properties, Pergamon Press, New York (1988), Gibson, L. J., Mat. Sci. and Eng A110, 1 (1989), Knippenberg and B. Lersmacher, Phillips Tech. Rev., 36(4), (1976), and Bonzom, A., P. Crepaux and E. J. Moutard, U.S. Pat. No. 4,276,246, (1981). Additives can be added to promote, or catalyze, the foaming, such as dissolved gases (like carbon dioxide, or nitrogen), talc powder, freons, or other standard blowing agents used in making polymer foams.
Then, unlike polymer foams, the pitch foam must be oxidatively stabilized by heating in air (or oxygen) for many hours, thereby, cross-linking the structure and xe2x80x9csettingxe2x80x9d the pitch so it does not melt, and deform the structure, during carbonization. See Hagar, Joseph W. and Max L. Lake, xe2x80x9cFormulation of a Mathematical Process Model Process Model for the Foaming of a Mesophase Carbon Precursor, Mat. Res. Soc. Symp., Materials Research Society, 270:35-40 (1992) and White, J. L., and P. M. Shaeffer, Carbon, 27:697 (1989). This is a time consuming step and can be an expensive step depending on the part size and equipment required.
Next, the xe2x80x9csetxe2x80x9d or oxidized pitch foam is then carbonized in an inert atmosphere to temperatures as high as 1100xc2x0 C. Then, a final heat treatment can be performed at temperatures as high as 3000xc2x0 C. to fully convert the structure to carbon and produce a carbon foam suitable for structural reinforcement. However, these foams as just described exhibit low thermal conductivities.
Other techniques may utilize a polymeric precursor, such as phenolic, urethane, or blends of these with pitch. See Hagar, Joseph W. and Max L. Lake, xe2x80x9cIdealized Strut Geometries for Open-Celled Foams,xe2x80x9d Mat. Res. Soc. Symp., Materials Research Society 270:41-46 (1992), Aubert, J. W., (MRS Symposium Proceedings, 207:117-127 (1990), Cowlard, F. C. and J. C. Lewis, J. of Mat. Sci., 2:507-512 (1967) and Noda, T., Inagaki and S. Yamada, J. of Non-Crystalline Solids, 1:285-302, (1969). However, these precursors produce a xe2x80x9cglassyxe2x80x9d or vitreous carbon which does not exhibit graphitic structure and, thus, has a very low thermal conductivity and low stiffness as well. See Hagar, Joseph W. and Max L. Lake, xe2x80x9cIdealized Strut Geometries for Open-Celled Foams,xe2x80x9d Mat. Res. Soc. Symp., Materials Research Society, 270:41-46 (1992).
One technique developed by the inventor of the present invention, and is fully disclosed in commonly assigned U.S. patent application Ser. No. 08/921,875. It overcomes these limitations, by not requiring a xe2x80x9cblowingxe2x80x9d or xe2x80x9cpressure releasexe2x80x9d technique to produce the foam. Furthermore, an oxidation stabilization step is not required, as in other methods used to produce pitch-based carbon. This process is less time consuming, and therefore, will be lower in cost and easier to fabricate than the prior art above. More importantly, this process is unique in that it produces carbon foams with high thermal conductivities, greater than 58 W/mxc2x7K.
The method described in U.S. patent application Ser. No. 08/921,875 can experience mold release problems when certain molds are used. For example, if a thick aluminum, steel, or graphite mold is used, the foam will crack in what appears to be tension failures at locations that were in contact with the mold prior to foaming. Thus a mold release is required, which can alter the final dimensions of the molded article, and require further processing, such as machining.
The invention described herein provides a method of casting a pitch based. The method includes forming a viscous pitch foam in a container. Transferring the viscous pitch foam from the container into a mold, and then hardening the viscous pitch foam to form a molded pitch based foam.
The general objective of the present invention is to provide a method of casting carbon foam without cracking caused by contacting the pitch to the mold prior to foaming. This objective is accomplishing by transferring the viscous pitch foam in the mold, and then hardening the viscous pitch foam to form a pitch derived foam. The pitch derived foam hardens in the mold without cracking because only the viscous pitch foam contacts the mold, and not the melted pitch.
Another objective of the present invention is to provide a method of casting carbon foam that does not require a mold release in the mold. This objective is accomplished by transferring the foamed pitch into the mold.
These and other objectives are accomplished by a method of casting a pitch based foam which includes the steps of: forming a viscous pitch foam in a container; transferring the viscous pitch foam from the container into a mold; and hardening the viscous pitch foam to form a cast pitch derived foam.